Chapter 32 / Sensory-Motor Integration in the Spinal Cord 767
Figure 32–3 The number of synapses in a reflex pathway
can be inferred from intracellular recordings.
A.An intracellular recording electrode is inserted into the cell
body of a spinal motor neuron that innervates an extensor mus-
cle. Stimulation of Ia sensory fibers from flexor or extensor mus-
cles produces a volley of action potentials at the dorsal root.
B.Left: When Ia fibers from an extensor muscle are stimulated,
the latency between the recording of the afferent volley and the
excitatory postsynaptic potential (EPSP) in the motor neuron is
only 0.7 ms, approximately equal to the duration of signal trans-
mission across a single synapse. Thus, it can be inferred that the
excitatory action of the stretch reflex pathway is monosynaptic.
Right: When Ia fibers from an antagonist flexor muscle are stim-
ulated, the latency between the recording of the afferent volley
and the inhibitory postsynaptic potential in the motor neuron is
1.6 ms, approximately twice the duration of signal transmission
across a single synapse. Thus, it can be inferred that the inhibi-
tory action of the stretch reflex pathway is disynaptic.
刺激
屈肌神经
大直径
传入
记录传入齐射
A 实验设置
B 推断通路中突触的数量
单突触
至伸肌
0.7 毫秒
刺激
伸肌神经
大直径
传入
运动神经元的
细胞内记录
双突触
1.6 毫秒
运动神经元中的
兴奋性突触后电位
传入齐射
5 毫秒
1 毫伏
system, can be demonstrated by selectively stimulat-
ing the alpha and gamma motor neurons under experi-
mental conditions.
When only alpha motor neurons are stimulated,
the firing of the Ia fiber from the muscle spindle pauses
during contraction of the muscle because the muscle
is shortening and therefore unloading (slackening) the
spindle. However, if gamma motor neurons are acti-
vated at the same time as alpha motor neurons, the
pause is eliminated. The contraction of the intrafusal
fibers by the gamma motor neurons keeps the spindle
under tension, thus maintaining the firing rate of
the Ia fibers within an optimal range for signaling
changes in length, whatever the actual length of the
muscle (Figure 32–5). This alpha-gamma co-activation is
recruited for many voluntary movements because it
stabilizes the sensitivity of the muscle spindles.
In addition to the axons of gamma motor neurons,
collaterals of alpha motor neuron axons sometimes
innervate the intrafusal fibers. Axons that inner-
vate both intrafusal and extrafusal muscle fibers are
referred to as beta axons. Beta axon collaterals provide
the equivalent of alpha-gamma coactivation. Beta
innervation in spindles exists in both cats and humans,
although it is unquantified for most muscles.
The forced linkage of extrafusal and intrafusal con-
traction by the beta fusimotor system highlights the
importance of the independent fusimotor system (the
gamma motor neurons). Indeed, in lower vertebrates,
such as amphibians, beta efferents are the only source of
intrafusal innervation. Mammals have evolved a mecha-
nism that frees muscle spindles from complete depend-
ence on the behavior of their parent muscles. In principle,
this uncoupling allows greater flexibility in controlling
spindle sensitivity for different types of motor tasks.
This conclusion is supported by recordings in spin-
dle sensory axons during a variety of natural movements
in cats. The amount and type of activity in gamma motor
neurons are set at steady levels, which vary according to
the specific task or context. In general, activity levels in
both static and dynamic gamma motor neurons (Figure
32–2B) are set at progressively higher levels as the speed
and difficulty of the movement increase. Unpredictable
conditions, such as when the cat is picked up or han-
dled, lead to marked increases in activity in dynamic
gamma motor neurons and thus increased spindle
responsiveness when muscles are stretched. When an
animal is performing a difficult task, such as walking
across a narrow beam, both static and dynamic gamma
activation are at high levels (Figure 32–6).
Thus, the nervous system uses the fusimotor system
to fine-tune muscle spindles so that the ensemble output
of the spindles provides information most appropriate
for a task. The task conditions under which independ-
ent control of alpha and gamma motor neurons occurs
in humans have not yet been clearly established.
The Stretch Reflex Also Involves
Polysynaptic Pathways
The monosynaptic Ia pathway is not the only spinal
reflex pathway activated when a muscle is stretched.
Type II sensory fibers from muscle spindles are also
activated. These discharge tonically depending on
Kandel-Ch32_0761-0782.indd 767 18/01/21 6:03 PM